What Have I Done?
by eternityisours
Summary: ONE SHOT. After Derek finds Mark and Addison in bed, Mark comes back to answer what Addison has been wondering all along. With one phrase, he sends her reeling. MarkAddison.


**Title: **What Have I Done?

**Disclaimer:** I don't own anything from _Grey's Anatomy_. But damn, how I wish I owned Patrick Dempsey!

**Rating: **M (mention of sex)

**Pairing:** Mark/Addison

**Summary:** ONE SHOT. After Derek finds Mark and Addison in bed, Mark comes back to answer what Addison has been wondering all along. With one phrase, he sends her reeling. Mark/Addison.

**A/N:** Only my second _Grey's Anatomy_ fic and my first Mark/Addison fic, so be gentle! Constructive comments/criticism on things like character development, wording, etc. would be great. This fic was written in a crazy, possessed haze, so again, please be gentle, and enjoy!

* * *

Fuck.

Addison Forbes Montgomery-Shepherd slammed her head twice against the oak headboard of her bed frame, not wincing or even blinking as it began to throb. She deserved it. She really felt that she did.

A day ago, things had been different. Hell, yesterday morning they had even been different. Yesterday morning, Derek hadn't hated Addison or Mark. Mark had still been his best friend, and Addison had still been his wife.

Technically, she _was_ still Derek's wife. But Derek was gone now, and she didn't even know where the hell he had went. Half of her wardrobe was still lying on the front steps sopping wet. There were still tears streaming down Addison's face, and she knew that she must look like shit. Nobody was there to see it, because nobody was there. Period.

It had happened on hours ago, Addison realized, even though it felt as if it had happened days ago. She glanced at the clock beside her. It was only three in the morning.

She'd been drunk, of course, and though she didn't quite remember how it had happened, she _did_ remember the sex. Oh, and Derek walking in.

She had been at a bar, where she was supposed to meet Derek after work, since he'd had a late surgery. Then she saw Mark sitting beside her and oh god, she couldn't even figure out why, but the next thing she knew they were kissing. It wasn't the quick, friendly kiss on the cheek that they usually shared, either. This was the real thing. It was hot and intoxicating and absolutely irresistible; the exact definition of Mark. Addison had an excuse for not being able to stop – she'd been drunk, and there had been a gorgeous man beside her. Mark, however, she was pretty sure, had been completely sober (which may have been a first for him).

The thing was, Addison hadn't _wanted_ him to stop. She knew it was wrong and that she was married and had responsibilities, but that just made it even more exhilarating and oh god, what had she been thinking? Surely sleeping with her husband's best friend would make him less distant. _Good move, Addison_, she chided herself.

But it was over now. She couldn't take back what she had done.

And Mark…god, Mark had just _been there_. She'd been lonely for so long, and he had been so comforting, so…so…he'd turned out to be everything that Derek wasn't. Mark had always gone to her when she was in need. He was supportive and Addison had gotten used to calling him if she was on the verge of a breakdown instead of Derek. She supposed that their relationship had begun long before they'd slept together, but she had never expected it to turn into anything.

Around other people, Mark was always witty and suggestive and quite the bastard, really. But around her, he was a completely different person. Somehow he became warmer – kinder. She didn't know why, until he told her the exact reason for it that night.

* * *

"Addison."

One o'clock in the morning and Addison is still sitting on those steps, replaying every moment in her head. Derek opening the door. Scrambling to put her clothes on. (She then realizes that the shirt she had put on is Mark's. And that she is still wearing it). Shoving Mark out of the house, and begging, _pleading_ with Derek to not go, to give them a chance. Derek telling her that they weren't Derek and Addison anymore. And the final blow – Derek leaving.

"Addison." The voice comes again, more forceful this time, cutting a path to her ears and forcing its way through her head.

"What?"

God, her voice. Even to her ears, it sounds strange and foreign. Addison's hair is sticking up, and the mascara that had run down her cheeks is dried up. Needless to say, her eyes are puffy and red. She is curled up, still in Mark's shirt, against the banister of the staircase.

"Addison…"

"_What_?" It comes out this time as a shout, and finally she pulls herself slowly, painfully, out of her stupor. It is as if everything is in slow motion, and Addison forces her eyes to drift upwards. "Mark, what the hell are you doing here?"

"I just wanted to tell you something," Mark begins, walking cautiously towards her, as if he thinks that at any moment she might burst into a fit of tears, or possibly rage.

"We screwed up, Mark," she replies blankly, returning her eyes to the spot on the wall that she has recently become very familiar with. Her body doesn't even take notice as Mark slowly sits down beside her, leaning against the wall. "We really fucked it up this time."

Addison doesn't usually cuss, but she tosses that word out like it is nothing, and that worries Mark. He doesn't know how she will react should he try to comfort her – this whole thing is, virtually, his fault.

"Is Derek still around?" He asks, glancing up the stairs into a dark hallway. The only light in the whole house is a dim glow coming from the lamp by the front door. It isn't far away, but the room seems to be even darker, even colder that it would usually be.

Addison lets out a bitter laugh.

"No," she answers. A long-awaited tear escapes from her eye and trails down the side of her cheek. "He's gone. He left. I – " her voice breaks, and she drops her head into her hands. "I don't know where he went."

"Addison," Mark whispers, placing his fingers under her chin and gently tilting her face up to look at him. Damn, she is beautiful. Even when there are dried-up darkly tinted tears on her face, even when she is at her lowest, even when –

_Oh, shut up_, Mark tells himself, disgusted. _She just lost her husband because of you, and all you can think about is her appearance?_

"He…Derek wasn't good enough for you," he finishes quietly.

At this, Addison whips her head up and out of Mark's hands. She stands up for the first time in hours; her muscles scream in protest, but she brushes the pain aside.

It takes a split second for him to figure out that this is the wrong thing to say.

"What?" She demands, eyes flashing. Addison had been so pale, so emotionless and lost since Mark had arrived that this startles him. She draws herself up to her full height and glares down upon him.

"Addison, come on." Mark's voice is rising along with hers. She knew that Derek wasn't; why was she denying it?

"No." Addison's voice is firm. "You don't know the first thing about Derek and I. what we had – "

"What you had was gone long before you decided to cheat on him," Mark shouts, now standing up as well.

She isn't so tall anymore.

"Shut up," Addison says, now in a whisper. "Don't you dare come in here and – "

"I am," he continues, drowning out her voice. "I can and I am. You know that your relationship with Derek was over a long time ago!"

Addison now stands with her back pressed against the wall, Mark towering over her. Somehow, in the process of them arguing, they've ended up in this compromising position; Mark grasping her arms, leaving not an inch of space between them.

She suddenly becomes aware of how close they are. He is breathing heavily, his eyes boring into hers, and she can feel his body against hers.

"Addison," Mark says, leaning closer to the point where she can feel his lips moving before she hears the words come out of his mouth. "I love you."

Time stops. Addison freezes.

"Wh – what?" she stutters, not fully comprehending what he has just said.

He doesn't even give her time to.

Mark's lips crash into hers, and suddenly Addison is falling, falling into the same whirlpool of ecstasy that she has experienced only hours ago.

But she won't let herself.

"Mark," she says, and it comes out muffled against his mouth. Addison places her hands on his chest and pushes, gently. He has no response but to pull her closer. She pushes him again. This time not so gently.

"What?" he says somewhat impatiently. His hand is already under her shirt, running up and down her bare back, almost rendering the part of Addison's brain that controls speech and language useless. But not quite.

"We can't do this." Addison steps away from Mark, and his hand drops down to meet his side.

"You can't – " and her voice breaks, because she knows what she is about to say is a lie. "You can't love me. You _don't _love me."

"Addison – " Mark begins, only to be cut off.

"No," her voice is more forceful. "I was lonely, and you were lonely, and we – we were both lonely. It can't happen again."

Her back is now facing him, and she lowers her gaze to the ground.

"You should leave, Mark," she finishes quietly.

A solitary tear drips down Addison's face as she feels Mark come closer, stopping as his chest touches her shoulder. Addison doesn't' move as he stoops so that his mouth is touching her ear.

"I love you," he whispers for the second time that night, and the mere feeling of his lips is enough to send Addison back into that whirlpool.

But the feeling disappears as Mark strides to the door, opens it, and disappears into the awaiting darkness.

* * *

**A/N:** Love it? Hate it? Either way, please review! Reviews with what you liked and didn't like (specifically – line by line, even, if you're generous enough!) are definitely appreciated. 


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